as yet vs up to now

as yet and up to now both are adverbs.

Word	Noun	Adjective	Verb	Adverb
as yet	No	No	No	Yes
up to now	No	No	No	Yes

As adverbs, as yet and up to now are synonyms defined as:

as yet and up to now: used in negative statement to describe a situation that has existed up to this point or up to the present time

Other synonyms of as yet include heretofore, hitherto, so far, thus far, til now, until now, yet.

as yet (adverb)	up to now (adverb)
used in negative statement to describe a situation that has existed up to this point or up to the present time	used in negative statement to describe a situation that has existed up to this point or up to the present time
	prior to the present time